LONDON — Sources within the campaign for another Brexit referendum have told Business Insider that they must get Jeremy Corbyn on board within the next few weeks or it is "game over" for the prospect of forcing a second vote.

Sources campaigning for a People's Vote said they estimate that there are currently just 175 out of the House of Commons' 650 MPs who currently support another referendum.

This includes 116 Labour MPs, some of whom are shadow Cabinet ministers. But crucially, not the Labour leader.

"We need to move Jeremy Corbyn or it's game over," one of the campaign’s leading figures told Business Insider.

Although the Labour Party is officially still committed to leaving open the prospect of a second referendum, so far Corbyn has been reluctant to actively support one.

The Labour leader travelled to Yorkshire on Thursday to set out Labour's position on Brexit. However, despite the best hopes of pro-Europeans in his party, there was little sign of him shifting Labour's position on the issue.

Instead Corbyn simply repeated the party's policy of forcing a snap general election, winning it, and then going ahead with Brexit on terms negotiated by a newly-elected Labour government.

With polling suggesting the vast majority of Labour's members are against Brexit and behind a new referendum, Corbyn's reluctance is incredibly frustrating for Labour voices within the People's Vote campaign.

Tom Baldwin, who previously worked for Ed Miliband and now heads up the People's Vote communication operation, told a campaign rally in London on Friday that the party had received thousands of letters calling for Labour to commit to stopping Brexit.

The Brexit dividend has been for the Royal Mail with all the post going to Corbyn.

Baldwin claimed that Labour's National Policy Forum has received over 13,000 emails and letters urging Corbyn to oppose Brexit. People's Vote campaigners say this is more than the party ever received on the Iraq war.

A Labour source told BI that the comparison with Iraq was "spurious," but the issue is undoubtedly sparking huge disagreements in the party. A Labour delegate involved in putting together the party's conference motion on Brexit told the same rally: "The Brexit dividend has been for the Royal Mail with all the post going to Corbyn."

However, despite growing pressure, Corbyn — guided by internal polling indicating little appetite for another referendum in its target constituencies — is showing no signs of declaring support for another public vote.

Figures in the People's Vote campaign say that while some shadow ministers privately support reversing Brexit, they are tightly-controlled by Corbyn's office, which contains Eurosceptics and has generally been unwilling to talk to anti-Brexit groups.

"There are people around Corbyn who think Brexit will be good for workers. They genuinely believe that,” they added, referring to figures in the Labour leader’s inner-circle.

A senior Labour MP on the bankbenches disputed the claim that only 175 MPs support a People's Vote.

They said "there are substantial numbers of MPs — notably on the Labour side who want to see the deal defeated before setting out their view on the best way forward.

"Many on the Conservative side feel duty bound to let the Prime Minister put her deal to Parliament — but when it becomes clear it cannot proceed — many have no doubt that the only way to resolve the impass will be to put the issue back to the people."

Fears that Corbyn will back May's deal

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There is another reason why there is a growing sense of urgency among People's Vote campaigners. Not only is time running out for a new vote, but it could be a matter of time before May produces a deal that Corbyn can support.

One school of thought among anti-Brexit campaigners and Labour MPs is that Corbyn will ultimately tell his MPs to back May's deal if a permanent customs union is added.

They may be right. As one senior aide to Corbyn admitted to BI last month: "I could see a scenario where May comes back with a slightly softer deal and we say 'well that is the best we can do. That nearly meets our objectives' and then we will try to renegotiate in office."

Despite this, May holds out hope that Labour backbenchers will move in her favour and this week held talks with those MPs who the government believes are the most likely to be persuaded to back her deal. The prime minister tried to woo them with guarantees on workers' and environmental rights, while Channel 4 reported on Thursday that a permanent customs union also came up in conversation.

BI has also learned that at least one government whip suggested the idea of a permanent customs union — a key plank of Labour Brexit policy — being added to the political declaration on the future UK-EU relationship as a means of winning opposition votes for the deal.

Intriguingly, one Labour MP told BI that a government whip had asked them whether "[the push for a customs union] would be better to come from the government or parliament," suggesting that government had an eye on customs union amendments for legislation like the Trade Bill.

Norway calling

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Corbyn isn't the only hurdle, however. Based on the People's Vote campaign's current estimates, there are still around 140 Labour MPs who don't support another referendum, with many already signed up to rival plans.

A People's Vote insider admitted that the "Norway Plus" soft Brexit option — chastised in People's Vote material — has "huge surge potential" in that many tens of MPs could come out for it as the March 29 cliff-edge approaches.

Under this model, the UK would effectively remain in the single market and form a new customs union with the EU. Owen Jones, Guardian columnist and influential voice on the left-wing of the Labour Party, reluctantly supported the Norway plus option on Thursday, describing it as the Labour Party's "least worst option."

[Brexit is like] half a horse. You can either have the head or the arse. But ultimately it's going to bleed to death.

One of its proponents, Labour MP Stephen Kinnock, told BI "it's attracting support from the right across the Labour movement, because it is becoming increasingly clear that it’s the only way to re-unite our deeply divided country."

He added: "Common Market 2.0 delivers on Labour’s call for a customs union and a strong single market deal, enables safeguards on the free movement of labour, and Norway has almost the highest level of state aid in Europe."

Other Labour MPs, who campaigned passionately for Remain in 2016, say that they'd rather focus on making the best of Brexit by pressuring the prime minister into an improved deal than re-open the wounds of two years ago.

Some Labour MPs believe that Labour should focus instead on domestic issues like rough sleeping and social care, and then wait for voters to see damaging impact of Brexit, before campaigning to rejoin.

One backbencher told BI that Brexit is like "half a horse." They added: "You can either have the head or the arse. But ultimately it's going to bleed to death."

With the Commons almost certain to vote down May's deal next week, and MPs set to debate alternatives in the days that follow, campaigners for a second referendum realise that it could be now or never for their dreams of forcing a second vote.

And without Corbyn's support soon, that dream could very quickly slip away.